1. Summary of HER2
ErbB2, also known as HER2/neu, is the second member of the EGFR family, which forms a heterodimer with other three members in the EGFR family, so as to exert the biological function. To date, the ligand which can directly bind to ErbB2 has not yet been found. The neu gene encoding ErbB2 was firstly separated from rat neuroblastoma. A homologous gene of neu gene in human somatic cells, known as HER2, is located on chromosome 17q21.1. The encoded product is ErbB2, which is consisted of 1255 amino acids with the molecular weight around 185 kDa, in which positions 720-987 belong to tyrosine kinase active domain. In addition to playing a role through PI3K and MAPK signaling pathway, ErbB2 may also reduce the expression of cyclin D and c-myc, thereby reduce the expression of the cyclin-dependent kinase (cdk) inhibitor p27kipl, leading to cell proliferation resulting from the inhibition of cdk2 activity [1].
With the increasingly expanding and deepening studies, it has been found that HER2 is expressed or over-expressed in various tumors. So far, it has been reported that the positive expression rate, over-expression rate of HER2 in several tumors and the number of people within whom HER2 is over-expressed are as follows: ovarian cancer, 45%, 21%, 23316 persons [2]; breast cancer, 58%, 38%, 223112 persons [3]. Hence, there is an urgent need in clinical practice for effective drugs targeting HER2 to treat malignant tumor. Till present, the commercially available monoclonal antibodies which target HER2 include Trastuzumab and Pertuzumab.
2. Trastuzumab and Pertuzumab
Herceptin® (Trastuzumab), developed by Genentech, is a humanized monoclonal antibody which targets HER2. In 1998, Trastuzumab in combination with paclitaxel was approved by the US FDA as a first-line therapeutic regimen for treating HER2/neu over-expressed metastatic breast cancer, or as a single drug for treating HER2/neu over-expressed metastatic breast cancer which had been subjected to at least one cycle of chemotherapy. Trastuzumab not only has high affinity with HER2 receptor, but also solves the problem of immunogenicity caused by applying murine-derived antibody to human body. The results of the clinical trials show that, using Trastuzumab only has the effective rate of 11.6%˜16%, while its combination therapy with chemical drugs has the effective rate up to 50%. Comparing with chemotherapy only, said combination confers patients with advanced recurrent breast cancer a longer life span and a reduced mortality.
Another antibody drug which targets HER2 is Pertuzumab [4], which was also developed by Genentech. Pertuzumab binds to region II of the extracellular domain of HER2 receptor, inhibiting the formation of a dimer, thereby inhibiting receptor-mediated signaling pathway, while Trastuzumab (Herceptin) binds to region IV of the extracellular domain of HER2 receptor. Pertuzumab was approved by US FDA on Jun. 8, 2012, for treating HER2-positive advanced metastatic breast cancer patients (see CN101023100B).
3. Antibody-Drug Conjugates
Monoclonal antibody has received more and more attentions due to its characteristics of high target specificity, low side effect and the like. However, when used alone, its therapeutic effect is limited. Now, the most successful monoclonal antibody drugs against tumor are the ones against lymphocytoma such as chronic non-Hodgkin's lymphocytoma (NHL). The Phase II clinical study of Rituxan directed to NHL shows that the total response rate was only 6%. Regarding Herceptin® against metastatic breast cancer, only 15% have a response. Therefore, most of the monoclonal antibody drugs are used in combination with chemotherapy. For example, Rituxan is used in combination with standard chemotherapy for treating chronic lymphocytoma, which may increase the effective rate up to 90%. Till present, the major way for increasing the therapeutic effect of monoclonal antibody is antibody-drug conjugates.
Antibody-drug conjugate belongs to a type of new anticancer “biological missile” drug, which is consisted of three parts: antibody, cytotoxin and the linker which links the two parts. Monoclonal antibody is coupled with cytotoxin through chemical coupling. Then the antibody-drug conjugate specifically recognizes the receptor on the surface of cancer cells and binds to the receptor by using the targeting of the monoclonal antibody, and then, the conjugate enters into the cells, and prevents cancer cells from proliferating and kills cancer cells by using proteases in cells which may release the cytotoxin. The antibody-drug coupling technology makes the small molecule drug and biological protein fuse together, which may have the advantages of both and increase the potency of drugs remarkably, reduce the side effects and thus turn into a new generation of therapeutic product.
The first successful example in clinical practice of targeted antibody-drug conjugate is Gemtuzumab ozogamicin (Wyeth, trade name: Mylotarg). Mylotarg is the firstly approved monoclonal antibody-drug conjugate. This drug is consisted of anti-CD33 antibody, Calicheamicin (a drug which degrades DNA) and chemical linker AcBut. Mylotarg is a drug in which a humanized anti-CD33 IgG4 is coupled with antitumor drug Calicheamicin, for treating acute myeloid leukemia [5]. Mylotarg is the first generation of antibody-drug conjugate, which has three critical defects in technology: firstly, the linker used to link toxin is very unstable, the half-life of which is only two days, leading to a severe toxin dropping and a high toxic side effect in clinical practice; secondly, the antibody is coupled with the linker through the amino group of lysine, nevertheless, there are tens of lysines on the surface of an antibody, and the coupling sites are random, which partly affect the potency of drugs; more importantly, since the coupling technology is not mature at that time, only 50% of antibodies may couple with the drug, which results in an unsatisfactory potency of drugs in clinical practice; thirdly, the antibody used is IgG4, which lacks antibody-dependent cell-mediated cytotoxicity (ADCC) and complement dependent cytotoxicity (CDC). Therefore, ten years after marketing, Mylotarg was withdrawn from the market due to a high toxic side effect and limited therapeutic effect.
The second successful example in clinical practice of targeted antibody-drug conjugate is a novel drug for treating Hodgkin's lymphoma. Since having a very good therapeutic effect, it was approved by US FDA in 2011 after only conducting Phase II clinical trial. This drug was developed by Seattle Genetics as a novel targeted antibody-drug conjugate (ADC), which is a targeted treatment for two types of patients with lymphoma expressing CD30 antigen. This antibody-drug conjugate, brentuximab vedotin, is consisted of monoclonal antibody against CD30, microtubule inhibitor (MMAE) and a dipeptide chemical linker. This antibody-drug conjugate has characteristics of low side effect and effective lymphoma inhibition. In a Phase II single group clinical trial, 102 patients from 15 to 77 years old (median age=31) with recurrent or refractory Hodgkin's lymphoma, received brentuximab vedotin treatment, and the median treatment comprises 9 cycles. When the median treatment course was 6.7 months, the overall response rate was 73%. When the median treatment course was 20.5 months, the complete response rate was 34%; 40% of patients who received the treatment achieved partial response [7]. The most common adverse reaction is peripheral nerve lesion. The success of this drug suggests a technological feasibility and a very bright future of the targeted antibody-drug conjugate.
Another successful example of targeted antibody-drug conjugate is T-DM1 against malignant breast cancer developed by Genentech Inc. [8]. The monoclonal antibody in this antibody-drug conjugate is an anti-HER2 (ErbB2) antibody on the surface of breast cancer cells, which is coupled with the cytotoxin, microtubule inhibitor DM1. The result in Phase III clinical trial of this drug shows better therapeutic effect than chemotherapy, and a lower side effect. Those patients with breast cancer, who have received Herceptin and taxanes chemotherapy drugs treatment previously, still have progressed diseases. However, receiving antibody-drug conjugate treatment may significantly prolong the survival time of those HER2-positive breast cancer patients under the premise of the disease not becoming worse [9]. Based on the good therapeutic effect of this drug, the drug was approved by US FDA on Feb. 22, 2013, for treating HER2-positive advanced metastatic breast cancer patients (see CN100482281C).
Although Herceptin is a breakthrough in the history of the treatment of HER2-overexpressed breast cancer in which various anticancer treatments have already been attempted, about 85% of the subjects have no or only weak response to Herceptin therapy[11]. It has shown that, HER2 is expressed or overexpressed in various tumors. Thus, there is an urgent need in clinical practice for developing anticancer drugs targeting HER2, for those patients with HER2-overexpressed tumors or other HER2-expressed relevant diseases (not just including breast cancer), who have no or only weak response to Herceptin therapy.
Therefore, there is an urgent need in clinical practice for developing a drug targeting HER2. The present invention provides a technical solution to meet this need.